Exercising three times a week reduces the odds of developing depression by around 16 per cent, scientists have found.

In a study conducted as part of a public health research consortium, the UK-based scientists studied 11,135 people and found those who increased their weekly activity reported fewer depressive symptoms.

The findings showed exercising three times a week reduced the risk, and each additional activity session per week further reduced odds by 6 per cent.

"Assuming the association is causal, leisure time physical activity has a protective effect against depression," said Dr Snehal Pinto Pereira of University College London's (UCL) Institute of Child Health, who led the study.

"If an adult between their twenties and forties who isn't physically active became active three times per week, they would reduce their risk of depression by approximately 16 per cent."

The scientists noted that the link between exercise and depressive symptoms was seen across the whole population, and not just in those at high risk of clinical depression.

The study also found that people who reported more depressive symptoms than others at age 23 tended to also be less physically active, but this link weakened as they grew older.

"This finding is important for policies designed to get people more active, because it suggests that depressive symptoms could be considered a barrier to activity in young adulthood," Dr Pereira said.

UCL professor of epidemiology and public health Chris Power said it added weight to existing evidence suggesting exercise could be used as a treatment for depression as well as boosting physical health.

"If everyone was physically active at least three times a week we would expect to see a drop in depression risk, not to mention the benefits for physical health," he said.

Depression is one of the most common forms of mental illness, affecting more than 350 million people worldwide. It is ranked by the World Health Organisation as the leading cause of disability globally.

Treatment for depression usually involves either medication or psychotherapy, or a combination of both, but many patients fail to get better and suffer recurring bouts of illness.